This invention relates to a circuit for controlling an asynchronous machine which is fed by a frequency converter (static frequency changer) having a line-controlled rectifier connected to a three-phase network, an intermediate DC link and a self-commutated inverter. More particularly, the invention relates to such a circuit in which a current control, having a function generator and a current regulator for the stator current, controls valves of the line-commutated rectifier to a desired value derived from an input variable fed to the function generator, the level of the flux being given. A parallel control system is provided for the stator frequency and comprises an actual-value computer and a load state control for supplying a frequency correction variable to the control unit of the self-commutating converter in accordance with the difference between a load state variable calculated from the actual values of the stator current and the stator voltage and a desired load state value calculated from the input variable of the function generator.
An arrangement of the above type can be used to predetermine a given torque for an asynchronous machine by setting an appropriate input variable into the input of the function generator. The arrangement can also be used for controlling the speed of an asynchronous machine by means of a speed control having an output control difference which is fed to the function generator input via a controller.
An arrangement of this type is described in Siemens-Zeitschrift 45 (1971), at pages 195 to 197. There, the electrical torque of the asynchronous machine is used as the load state variable. An actual value of this torque is calculated from the instantaneous values of the stator current and the stator voltage in an actual-value computer. The output variable of a speed controller, into which the control difference of the stator frequency is fed, serves as the reference value. A reference value for the slip frequency is formed in a torque control in which the control difference between the calculated actual instantaneous value and the output variable of the speed controller corresponding to the desired instantaneous value are fed. The correcting variable for the stator frequency, which addresses the control unit for the valves of the self-commutating converter, is formed from the reference value in the slip frequency in a subsequent computing stage, taking into account the actual speed value of the asynchronous machine. Further, the output of the speed controller is fed to a function generator which furnishes, from a predetermined (set-in) characteristic, the reference value for the DC current in the intermediate link. The characteristic of the function generator is set so that the motor operates with a constant, rated flux and therefore can develop full torque at any speed. A secondary current controller forms a control variable for the control unit controlling the valves of the line-commutated converter from this current reference value and from an actual current value taken from the intermediate DC link.
According to the underlying control concept, the stator frequency control causes the stator current vector i (the magnitude of which is given by the current control loop) to ultimately follow the rotating main flux vector .psi. in such a way that the stator current component parallel to the flux vector (the magnetizing current i.sub..phi.1) controls the magnitude of the flux to be held constant, and the stator current component perpendicular thereto (the active current i.sub..phi.2) controls the preset torque. However, the dependence of the torque on the angle between the main flux vector and the stator current vector (the load angle) is maximum at 45.degree.. For load angles above 45.degree., a load at the shaft of the asynchronous machine, which is beyond the actual value of the torque and tends to lower the speed, also leads to a further reduction of the torque delivered and, ultimately, to stopping the machine (falling out). For protection against falling out at load angles above 45.degree., it is therefore necessary in the known arrangement to linearize the characteristic in the actual-value computer; this is not detailed, however. In this control concept, moreover, weakened-field operation is not indicated and is possible only at additional cost.
The present invention provides, in an arrangement of the above-mentioned kind but through the use of a different load state variable, an improved control concept which is fallout-proof at load angles above 45.degree. and which allows simple steps for further improvement, depending on the respective application. Thus, operation with field-weakening can be carried out easily, and measures for stabilization (damping of oscillations) and/or for pullout protection at load angles of up to nearly 90.degree. involve only slight extra cost. This is of advantage, for instance, where blowers, centrifugal pumps, stirrers or centrifuges are being driven. A tachometer generator for measuring the speed is not necessary. The use of tachometer generators is not precluded, however; and such may be used for more stringent requirements.